GB2027903A

GB2027903A - Detecting size and shape of bodies capacitatively

Info

Publication number: GB2027903A
Application number: GB7923055A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1978-08-08
Filing date: 1979-07-03
Publication date: 1980-02-27
Also published as: JPS5524693A; DE2932184A1; FR2433170B1; GB2027903B; IT1119129B; FR2433170A1; IT7968633A0; US4284947A; DE2932184C2

Abstract

The size and shape of a body is determined by rolling it between the plates of capacitors (3, 4; 6, 7; 10, 11), and measuring the capacitance changes. A capacitor (3, 4) comprising two parallel, spaced wires inclined to the rolling direction and above and below the rolling body scans sections of the body along its longitudinal axis, another (6, 7) determines the body's lengths and a third (10, 11) comprising two non-parallel wires determines the position of the body. The capacitance changes are compared with those produced by a body of known size and shape so that the size and shape of the body can be determined. During measurement, capacitor 3, 4 is initially used as a reference for capacitor 6, 7, and thereafter capacitor 6, 7 is used as a reference for capacitors 3, 4 and 10, 11. <IMAGE>

Description

1 0 1 5 GB 2 027 903A 1

SPECIFICATION

Detecting the size and shape of bodies The invention relates to the detection of the 70 shape and size of bodies.

According to one aspect of the present invention, a method of detecting the shape and size of a body comprises moving the body between the plates of a capacitor, detecting the changes in dielectric constant caused by movement of the body, comparing the changes so detected with those caused by a body of known size and shape and deducing from the comparison the size and shape of the 80 body.

Preferably, the position of the body in rela tion to the plates is determined.

Advantageously, the body is moved be tween the plates by rolling thereof.

According to another aspect of the present invention, apparatus for detecting the shape and size of a body comprises a capacitor, between the plates of which the body is movable, circuit means for determining changes in dielectric constant of the capacitor caused by movement of the body between the plates and comparator means for comparing said changes in dielectric constant caused by a body of known size and shape thereby to determine the size and shape of the body.

Preferably, the capacitor comprises parallel wires.

Advantageously, the apparatus comprises a slope whereon the body is moved by rolling. 100 Conveniently, the parallel wires are dis posed across the slope.

Conveniently, the wires are longer than the maximum circumference of the body.

Advantageously, the apparatus comprises means for determining the position of the body between the plates of the capacitor.

Preferably, the means comprises a second capacitor.

Advantageously, the apparatus comprises means for determining if the maximum length of the body exceeds a preselected level. Con veniently, the means comprises a third capaci tor.

An embodiment of the present invention 115 will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic side view of apparatus for performing a method of detecting the size and shape of a body.

Figure 2 is a plan view of the apparatus shown in Fig. 1, and Figure 3 is an incomplete electrical circuit -60 diagram partly in block form for detecting and processing changes in electrical parameter occurring in the apparatus of Figs. 1 and 2.

The invention is conveniently used for detecting the size and shape of right cylindrical pellets intended for insertion in nuclear fuel pins. Such pellets may, for example, comprise polymethyimethacrylate resin sold under the Registered Trade Mark---PERSPEX-or brass and uranium oxide.

In Figs. 1 and 2 there is shown a slope 1 having channel 2 down which the pellets to be examined roll. The slope is inclined at an angle 0 to the horizontal and the value of 0 is chosen so that pellets roll down the slope at satisfactory rate without bouncing from the surface of the base of the channel. A convenient value for 0 is about 1 W.

The base of the channel 2 supports an oscillator wire 3 to which an alternating carrier signal is applied. A detector wire 4 is supported above the channel and runs parallel to the oscillator wire 3. The wires 3 and 4 form the plates of a capacitor. A pellet 5 rolling down the channel passes between the wires 3, 4 which are inclined across the channel 2. The length of the channel 2 is such that the pellet undergoes several revolutions whilst between the wires 3, 4. As the pellet rolls down the slope the cylindrical surface is progressively exposed to the wires 3, 4 and the surface is scanned by the capacitor. A convenient channel length is about fifteen times the pellet circumference. The rolling pellet causes changes in the die- lectric constant of the capacitor system and the change is monitored by apparatus such as that described with reference to Fig. 3 below and Fig. 3 of British Patent Specification 1517364. The capacitor formed by the wires 3, 4 would be connected to the point B in the last mentioned Figure and a suitably-sized reference capacitor would be connected to the point C.

The capacitor formed by wires 3, 4 may be used to detect defects in the surface of the pellet along its longitudinal axis, differences in diameter and differences in the shape of the pellets. From the nature of the changes in capacitance it may be possible to detect if the pellet is non-circular in cross-section or if its cross-section varies along the length of the pellet as would occur if the pellet were tapered or in the shape of a barrel (increased diameter at the centre) or diabolo (increased diameter at the ends). Additional means to be described hereinafter are required to detect variations in the length of the pellets, and in pellets of annular cross-section to detect faults such as total or partial blocking of the central aperture or non-central location of the aperture. In Figs. 1 and 2 there are shown two capacitance probes 6, 7 located on opposite sides of the channel in the slope at a higher level than the wires 2, 3. As a pellet passes between the probes one of which, is supplied with an alternating carrier signal, the dielectric constant of the capacitor system formed by the two probes 6, 7 changes. By comparison of the changes with those obtained when a standard pellet of known length and configu- 2 GB 2 027 903A 2 ration passes between the probes it is possible to detect pellets which differ from the standard and reject those which fall outside the chosen tolerance limits. The carrier signal applied to the one of said probes may be the same as that applied to the wires 2, 3.

Alternatively it may be desirable to utilise a different carrier signal for the probes such as one with a higher peak to peak oscillating voltage. In the former case the capacitor systems formed by the wires 2, 3 and the probes 6, 7 may be connected in series to point B of the apparatus illustrated in Fig. 3 of British Patent Specification 1517364. In the latter case the capacitor systems formed by the wires 2, 3 and the probes are coupled to two of the arms of a four terminal network having four arms (that is one of them is coupled to B and the other to C of the abovementioned Fig. 3). This arrangement is illustrated in Fig. 3 of the present application and will be described below.

If the pellets were all sufficiently similar that the time they took to roll down the slope was substantially the same for each one then the comparison of their shape along their longitudinal axis could be made on the basis of the output signal at a particular time interval. However for pellets which show a divergence of rolling times the comparison is better made on the basis of the output signal when the pellet is at a particular point on the plane 1. A position detecting system is incorporated into the plane 1 as can be seen from Fig. 2 in which two wires 10, 11 (shown chain dotted) are located at the bottom of the channel 2. The distance between the wires 10, 11 decreases towards the bottom of the inclined plane 1 and so as the pellet rolls down the channel 2 the dielectric constant between the wires changes progressively. A carrier frequency different from that applied to the capacitor systems formed by the wires 2, 3 and probes 6, 7 must be used for the posi- a charge amplifier 16, the capacitance probe 7 to charge amplifier 17 and the wire 11 to a charge amplifier 18. A charge amplifier is a DC operational amplifier with a capacitive feedback which can produce a voltage output directly proportional to the change in charge at its input terminals. The output of charge amplifiers 16 and 17 are fed to a differential amplifier 19. The output from amplifier 17 is also tapped for feeding to a differential am plifier 20 as will be described below. The differential amplifier 19 has its output con nected to a bandpass filter 21 and then via a demodulator 22 and an amplifier 23 to a signal processing unit 25 in analysis and control circuitry 26.

The charge amplifier 18 has its output connected to the differential amplifier 20, the other input to the amplifier 20 being from charge amplifier 17 as mentioned above. The output of the differential amplifier 20 is to the signal processing unit via similar circuitry to that used in relation to the amplifier 19, in the case of amplifier 20 being via band pass filter 27, demodulator 28 and amplifier 30.

The signal processing unit 25 comprises comparators 31 and 32 for comparing values of input signals with stored values in memory unit 33. The memory unit 33 includes re membered and desired values of signals from the differential amplifiers 19 and 20 in units 34 and 35, respectively, operating in conjunc tion with comparators 31 and 32, respec tively. The memory unit 33 can be pro grammed with desired preselected values of signals or it can be programmed by running a reference pellet of desired size and shape characteristics down the slopel. The unit 25 can control a unit 37 provided for rejecting pellets of unsuitable shape, for example by effecting opening or closure of electrohydrau lic gates at the bottom of slope 1.

The unit 25 is connected to a data interface 38 so that data can be transmitted to central tion-detecting capacitor system comprising the 110 control or other processor, if necessary, and wires 10, 11 to eliminate cross-coupling ef fects. The output from the position-detecting capacitor system is passed to an independent amplifier. The use of position rather than time as a basis for the comparison of the output signal from a standard pellet and a pellet under examination eliminates differences caused by different rolling times.

Reference is now made to Fig. 3, wherein like reference numerals are used for like parts 120 in Fig. 1 and 2. A buffered driver signal at frequency f, for the capacitors 3, 4; 6, 7 is provided at differing magnitude for each ca pacitor, from an oscillator arrangement 14 the interface 38 is provided with a display 39 for example an oscilloscope or visual display unit so that the shape of pellets can be seen visually.

In operation of the circuitry of Fig. 3, suppose that pellet of unknown size and shape which may or may not be satisfactory is put at the top of the slope ready for rolling.

The memory unit 33 will already have been programmed with desired values of shape and a range of allowed values manually and also by rolling a reference pellet down the slope 1.

The unknown pellet is first checked for size in order that its length is not so great as to through a buffering amplifier arrangement 15. 125 render it unsatisfactory for nuclear use. This The capacitor 10, 11 receives a separate buffered driver signal at frequency f2 from oscillator arrangement 12 via buffering am plifier 13 to avoid interference between the detector systems. The wire 4 is connected to t first checking is effected by the capacitance probes 6 and7, since as the pellet rolls between the probes, the dielectric constant of the capacitor comprising probes 6 and 7 changes, thereby causing the voltage from 3 GB 2 027 903A 3 charge amplifier 17 to change which in turn causes a change in value of the output of amplifier 19 which varies the input into unit 26. The memory unit 33 contains a range of desired values of the signal in the unit 34 and if the memory signal from the amplifier 23 falls outside this range as determined by comparator 31, then the pellet is rejected. A reference value of capacitance for the differen- tial amplifier is provided by the charge amplifier 16 which has a constant output because the capacitor 3, 4 is not yet in use.

Assuming that the pellet has not been rejected then it will continue to roll down the slope 1 and will roll between the wires 3, 4. The part of the pellet, for the time being between the wires 3, 4 will determine the capacitance of the capacitor 3, 4. Reference to Fig. 2 will show that a different section of the pellet is between the wires 3, 4 depending on where the pellet is down the slope 1. Thus the measured value of capacitance is dependant upon the section of the pellet for the time being between the wires 3 and 4.

Consequently, the size and shape of the pellet at that section is determined because the voltage output of amplifier 19 changes and then the signal to circuitry 26 via filter 21, demodulator 22 and amplifier 23 changes.

The memory 34 contains known values of capacitance for known sizes and shapes and these are compared with the incoming signal in the comparator 31.

I n order that the size and shape of the whole pellet can be determined, it is necessary that information be provided as to how far down the slope the pellet has rolled. This information is provided by the capacitor constituted by the wires 10, 11 since as the pellet rolls down the slope, the capacitance of this capacitor (re dielectric constant) changes because progressively less of the pellet comes between the wires owing to their relative approach. Thus the capacitance of capacitor 10, 11 is a measure of how far down the slope the pellet has rolled. The change in capacitance causes a change in output of the amplifier 18 and also that of amplifier 20, thereby to feed a changed input signal into circuitry 26 from amplifier 30.

The capacitor 3, 4 is used initially as the reference capacitor for capacitor 6, 7 and thereafter the capacitor 6, 7 as reference capacitor for the capacitors 3, 4 and 10, 11.

This is possible because the value of capacitance of the capacitor 6, 7 varies initially and is then constant, whereas the value of capacitance of the capacitor 3, 4 is constant initially and then varies, there being only one pellet in the system at a time. This is due to the geometry of the slope layout as can best be seen in Fig. 2. A reference capacitor is used in this function when constant. Owing to the function of differential amplifier 19, the signal to the processing circuitry 26 will be of different polarity, dependent upon which of the capacitors 3, 4 or 6, 7 is being altered by the pellet. This difference in polarity enables the circuitry to deduce which of the capacitors is in operation. A similar situation obtains mutatis mutandis in the case of the capacitors 6, 7 and 10, 11.

The data as to size and shape of the pellets derived in the unit 25 can be transmitted to other data processing units by means of the data transmission interface 38. The size and shape of the pellets can be displayed on the display 39 for the convenience of operators.

In some embodiments of the invention, the nature of the material being scanned by the capacitors is determined.

Claims

1. A method of detecting the shape and size of a body comprising moving the body between the plates of a capacitor, detecting the changes in dielectric constant caused by movement of the body, and comparing the changes so detected with those cause by a body of known size and shape and deducing from the comparison the size and shape of the body.

2. A method as claimed in claim 1, in which the position of the body in relation to the plates is determined.

3. A method as claimed in claim 1 or 2, in which the body is moved between the plates by rolling thereof.

4. Apparatus for detecting the shape and size of a body comprising a capacitor, between the plates of which the body is movable, circuit means for determining changes in dielectric constant of the capacitor caused by movement of the body between the plates and caparator means for comparing said changes with changes in dielectric constant caused by a body of known size and shape thereby to determine the size and shape of the body.

5. Apparatus as claimed in claim 4, in which the capacitor comprises parallel wires.

6. Apparatus as claimed in claim 4 or 5, comprising a slope whereon the body is moved by rolling.

7. Apparatus as claimed in claim 6, in which the parallel wires are disposed across the slope.

8. Apparatus as claimed in claim 6, in which the wires are conveniently longer than the maximum circumference of the body.

9. Apparatus as claimed in any one of claims 4 to 8, further comprising means for determining the position of the body between the plates of the capacitor.

10. Apparatus as claimed in claim 9, wherein the means comprises a second capacitor.

11. Apparatus as claimed in any one of claim 4 to 10, further comprising means for determining if the maximum length of the body exceeds preselected level.

4 GB 2 027 903A 4

12. Apparatus as claimed in claim 11, wherein the means for determining maximum length comprises a third capacitor.

13. A method of detecting the shape and size of a body substantially as hereinbefore described with reference to the accompanying drawings.

14. Apparatus for detecting the shape and size of a body substantially as hereinbefore described and as shown in the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess 8- Son (Abingdon) Ltd.-1 980. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

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